1. Field of the Invention
This invention relates to high speed optical modulators and more specifically to linearized directional coupler modulators in which nonlinear distortion effects are suppressed.
2. Description of the Related Art
Nonlinear distortion effects are a major obstacle in the design of high speed signal transmission systems that utilize optical modulators. They set a practical limit to the dynamic range of the system. The lower end of the system's dynamic range is set by its noise floor, while the upper end is set by nonlinear effects, such as second-order harmonics and third-order intermodulation distortion (IMD).
A directional coupler modulator generally consists of two parallel waveguides that are fabricated on an electro-optic substrate in close proximity so that light launched into one waveguide (the reference arm) couples to the other waveguide (the signal arm) via evanescent coupling. If the waveguides have the same propagation constants, light launched into the reference arm will transfer completely to the signal arm in a distance 1=.pi./2.kappa., where .kappa.is the coupling coefficient which describes the strength of the interguide coupling.
Electrodes are placed over the waveguides in the coupler region and, when a voltage is applied across the electrodes, electric field lines normal to the substrate surface are oppositely directed in each waveguide. The oppositely directed electric fields produce a phase mismatch (or propagation constant mismatch) by increasing the refractive index in one guide and decreasing it in the other through the linear electro-optic effect. The degree of light transfer along a given length via evanescent coupling depends on the difference in propagation constants between the waveguides. Therefore, the optical switching can be controlled with the applied electric fields. A detailed description of this device can be found in R. V. Schmidt, "Integrated Optics Switches and Modulators," in Integrated Optics: Physics and Applications, ed. S. Martelluci and A. N. Chester (New York: Plenum Press, 1981), pp. 181-210.
Nonlinear distortion effects arise as a result of the nonlinear nature of the modulator's energy transfer curve, as illustrated in FIG. 1. FIG. 1 illustrates the energy transfer curve for a directional coupler modulator whose waveguides are two coupling lengths long. This causes the light that is launched into the reference arm to completely couple to the signal arm and back to the reference arm before exiting the coupler, when no voltage is applied to the electrodes. It is apparent from this graph that the amount of light that exits the signal arm varies nonlinearly with applied voltage. The nonlinear nature of the energy transfer curve results in nonlinear distortion of the output signal, primarily as a result of second harmonic distortion and third-order intermodulation distortion (IMD).
Linearized directional-coupler modulators, such as the ones described in Juan F. Lam and Gregory L. Tangonan, "A Novel Optical Modulator System with Enhanced Linearization Properties", IEEE Photo. Tech. Lett., vol. 3, No. 12 (1991), pp. 1,102-1,104 and in the related application mentioned above, are a special class of modulator that reduce nonlinear distortion effects.
This type of modulator, illustrated in FIG. 2, is similar to the standard modulators described above in that two parallel waveguides 10 and 12 are fabricated on a lithiumniobate electro-optic substrate 14 in close lateral proximity so that light 16 launched into one waveguide couples to the other waveguide via evanescent coupling. However, this type of modulator differs from standard modulators in that the electrodes are divided into an active section 18 and one or more passive sections 20 and 22. The active section 18 is used to modulate the optical beam 16 (with an RF source 24). Bias voltage sources 26 and 28 are used to apply DC bias voltages to passive sections 20 and 22, respectively. Proper biasing of passive sections 20 and 22 will result in a more linear energy transfer curve, which results in substantial suppression of second-order harmonics and IMD.
A problem associated with the linear directional coupler modulator is that the DC bias voltages applied to the passive sections must be very accurately controlled in order to suppress the nonlinear distortion effects. A phenomenon known as DC drift makes such accurate control difficult, especially when lithium-niobate is used as the substrate. Within minutes of setting the DC bias voltages at their optimum values, they begin to drift. This drift causes a degradation in the nonlinear distortion suppression.